A series of incidents involving Chinese tourists in Vietnam is keeping a deep maritime sovereignty dispute in focus despite official efforts to improve relations.

Last week 14 Chinese tourists wearing the same red-on-white T-shirts bearing a map of their county’s claim passed through immigration in the central Vietnamese province of Khanh Hoa.

Immigration agents confiscated the shirts as Vietnamese voiced outrage online toward the tourists. The backs of the shirts showed the nine-dash line that demarcates Beijing’s claim to the contested South China Sea, lopping off waters that Vietnam says are its own.

China has historically encouraged citizens to promote official foreign policy when overseas and particularly “correct world mis-impressions” of China, said Alan Chong, associate professor at the S. Rajaratnam School of International Studies in Singapore.

“All kinds of private activities coming out of China are seen as propaganda,” Chong said. “It’s all in their history.”